An acronym for the American Standard Code for Information Interchange, the agreed upon standard for representing characters (alphabetic, symbolic, etc.) in the memory of the computer.

Binary :

The numerical base upon which computer programs are modeled. Also referred to as base 2, it is a mathematical way of representing the electrical circuitry upon which computers operate. A 0 represents an off state, and a 1 indicates an on state. A combination of eight ons and offs are used to express bytes of program or data in memory.

See Also: Byte

Buffer :

A temporary data storage location in memory. As an example, a disk input-output buffer is an area of memory that stores data read from the disk to temporary locations while processing it.

Byte :

Eight bits of data, the smallest unit of information stored in the computer’s memory. As an example, one byte is required to represent one ASCII character.

Character :

A letter, digit, punctuation mark, or special symbol stored and processed by the computer.

See Also: ASCII String

Delimiter :

A character or other symbol that marks a boundary.

Destination :

The work area, file, or device to which data is sent.

Device :

Either an actual physical component of the computer system such as printer or a DOS handle that refers to it (e.g., PRN:), or a logical device that behaves and is addressed the same way as a physical device (e.g., a print spooler).

Disk :

A magnetic storage medium designed for long-term storage. Disks come in two varieties: hard disks (fast but fixed) or floppy disks (slow but removable). A disk can be partitioned into multiple volumes, each containing a tree-structured directory system that holds files accessible by programs.

See Also: Directory, File, Volume

Dynamic :

Used generically to refer to data or algorithms that change with time. Often used specifically to describe algorithms that automatically adjust to prevailing conditions.

Formfeed :

A special character (CHR(12)) that by convention causes most printers to move the printhead to the top of the next page.

See Also: Hard Carriage Return Linefeed

Hard Carriage Return :

An explicit carriage return character at the end of a line in a text file, as opposed to a soft carriage return that might be inserted into text by a program designed to handle word wrapping. A hard carriage return character is generated by the expression (CHR(13)) where a soft carriage return character is generated with the expression (CHR(141)).

Hexadecimal :

A representation of a value in base 16 rather than decimal which is base 10. Hexadecimal values are easily converted to and from binary (base 2) which is the form of data the computer actually uses. Hexadecimal values are represented by digits zero through nine and A through F for values between 10 and 15.

Extended Character Set :

The character set built into the ROM of the PC. This character set is a superset of ASCII, containing additional special characters (such as a line drawing character set) that may be used to enhance your program screens.

Linefeed :

A special character (CHR(10)) that by convention causes the cursor or printhead to move to the next line or to terminate a line in a text file. It is usually used in combination with a hard carriage return.

See Also: Formfeed, Hard Carriage Return

Port :

A designation for the hardware that allows the processor to communicate with peripheral devices.

Query :

A query is also a general term used when you want to interrogate a setting or an exported instance variable for its current value.

Queue :

A data structure of variable length where elements are added to one end and retrieved from the other. A queue is often described as first in, first out.

See Also: Print Spooler, Stack

Soft Carriage Return :

A carriage return that is introduced into text usually in order to implement some sort of wrap operation, as opposed to a Hard Carriage Return that was specifically entered into the text when it was created.

Stack :

A data structure of variable length whose elements are added and retrieved from the same end. A stack is often described as first in, last out.

Although it is a powerful general-purpose programming language, it was primarily used to create database/business programs. Harbour have been actively maintained looking for diversity keeping backward-compatible with Clipper style. It has undergone many changes and revisions and regain widely popularity amongst programmers in 1980s and 1990s

The open source Harbour license is similar to the GNU General Public License, with an exception supporting proprietary applications, so proprietary applications can be produced with Harbour and distributed..

History

The idea of a free software Clipper compiler has been floating around for a long time and the subject has often cropped up in discussion on comp.lang.clipper. Antonio Linares founded the Harbour project and the implementation was started.

Sailing the Clipper ship to a Harbour port. Clipper is a type of ship. Harbour is a synonym to port (where ship docks) Harbour is out port to the Clipper language.

On 2009 Harbour had a huge make over on its design promoted mainly by Viktor Szakáts and Przemyslaw Czerpak

Database support

Harbour extends the Clipper Replaceable Database Drivers (RDD) approach. It offers multiple RDDs such as DBF, DBFNTX, DBFCDX, DBFDBT and DBFFPT. In Harbour multiple RDDs can be used in a single application, and new logical RDDs can be defined from combination of other RDDs. The RDD architecture allows for inheritance, so that a given RDD may extend the functionality of other existing RDD(s). Third-party RDDs, like RDDSQL, RDDSIX, RMDBFCDX, Advantage Database Server, and Mediator exemplify some of the RDD architecture features. DBFNTX implementation has almost same functionality of DBFCDX and RDDSIX. NETIO and LetoDBprovide remote access over TCP protocol Harbour also offers ODBC support by means of an OOP syntax, and ADO support by means of OLE. MySQL, PostgreSQL, SQLite, Firebird, Oracle are examples of databases which Harbour can connect.

xBase technologies often is confused with a RDBMS software. Although this is true, xBase is more than a simple database system as the same time xBase languages using purely DBF can not provide full concept of a real RDBMS

Programming philosophy

Unlike Java which is intended to be write once, run anywhere, Harbour aims to be write once, compile anywhere. As the same compiler is available for all of the above operating systems, there is no need for recoding to produce identical products for different platforms, except when operating system dependent features are used. Cross-compiling is supported with MinGW32. Under Microsoft Windows, Harbour is more stable but less well-documented than Clipper, but has multi-platform capability and is more transparent, customizable and can run from a USB flash drive.

Under Linux and Windows Mobile, Clipper source code can be compiled with Harbour with very little adaptation. Most software originally written to run on Xbase ++, Flagship, FoxPro, xHarbour and others dialects can be compiled with Harbor with some adaptation. As 2010 many efforts have been made to turn the transition from other xBase dialects easier.

Harbour can make use of multiple Graphic Terminal emulations, including console drivers, and Hybrid Console/GUIs, such as GTWvt, and GTWvg.

Harbour supports external GUIs, free (e.g. HWGui, MiniGUI and Qt) and commercial (e.g. FiveWin, Xailer). HBQt is a library provinding bindings to Qt. HBIDE application included in official distribution and SVN repository is a sample of HBQt potencial.

Harbour is 100% Clipper-compatible and supports many language syntax extensions including greatly extended run-time libraries such as OLE, Blat, OpenSSL, FreeImage,GD, TIP, Tpathy, PCRE, HbZip (zlib and bzip2), cURL, Cairo, its own implementation of CA-Tools and NanFor libraries and many others. Harbour has an active development community and extensive third party support.

Any xBase language provides a very productive way to build business and data intensive applications. Harbour is not an exception.

Macro Operator (runtime compiler)

One of the most powerful features of xBase languages is the Macro Operator ‘&’. Harbour’s implementation of the Macro Operator allows for runtime compilation of any valid Harbour expression. Such a compiled expression may be used as a VALUE, i.e. the right side of an assignment (rvalue), but more interestingly, such a compiled expression may be used to resolve the left side (lvalue) of an assignment, i.e. PRIVATE, or PUBLIC variables, or a database FIELD.

Additionally, the Macro Operator may compile and execute function calls, complete assignments, or even list of arguments, and the result of the macro may be used to resolve any of the above contexts in the compiled application. In other words, any Harbour application may be extended and modified at runtime to compile and execute additional code on-demand.

Latest Macro compiler can compile any valid Harbour code including code to pre-process before compile.

Syntax:

&( ... )

The text value of the expression ‘…’ will be compiled, and the value resulting from the execution of the compiled code is the result.

&SomeId

is the short form for &( SomeId ).

&SomeId.postfix

is the short form of &( SomeId + “postfix” ).

Object Oriented Programming

Programming in an OOP style is a broader issue than a specific library or a specific interface, but OOP programming is something many Clipper programmers have come to expect. CA-Clipper 5.2 and especially 5.3 added a number of base classes, and a matching OOP syntax. Libraries such as Class(y), Fivewin, Clip4Win, and TopClass provide additional OOP functionality.

Harbour has OOP extensions with full support for classes including inheritance, based on Class(y) syntax. OOP syntax in Harbour is very similar to that of earlier Clipper class libraries so it should be possible to maintain legacy Clipper code with minimal changes.

Syntax and semantics

Harbour as every xBase language is case insensitive and can optionally accept keywords written just by first four characters

Built-in data types

Harbour has 6 scalar types : Nil, String, Date, Logical, Number, Pointer, and 4 complex types: Array, Object, CodeBlock, and Hash. A scalar holds a single value, such as a string, number, or reference to any other type. Arrays are ordered lists of scalars or complex types, indexed by number, starting at 1. Hashes, or associative arrays, are unordered collections of any type values indexed by their associated key, which may be of any scalar or complex type.

Hashes may use any type including other Hashes as the Key for any element. Hashes and Arrays may contain any type as the Value of any member, including nesting arrays, and Hashes.

Codeblocks may have references to Variables of the Procedure/Function>method in which it was defined. Such Codeblocks may be returned as a value, or by means of an argument passed BY REFERENCE, in such case the Codeblock will “outlive” the routine in which it was defined, and any variables it references, will be a DETACHED variable.

Detached variables will maintain their value for as long as a Codeblock referencing them still exists. Such values will be shared with any other Codeblock which may have access to those same variables. If the Codeblock did not outlive its containing routine, and will be evaluated within the lifetime of the routine in which it is defined, changes to its Detached Variables(s) by means of its evaluation, will be reflected back at its parent routine.

Codeblocks can be evaluated any number of times, by means of the Eval( BlockExp ) function.

Variables

All types can be assigned to named variables. Named variable identifiers are 1 to 63 characters long, start with [A-Z|_] and further consist of the characters [A-Z|0–9|_] up to a maximum of 63 characters. Named variables are not case sensitive.

Variables have one of the following scopes:

LOCAL: Visible only within the routine which declared it. Value is lost upon exit of the routine.

STATIC: Visible only within the routine which declared it. Value is preserved for subsequent invocations of the routine. If a STATIC variable is declared before any Procedure/Function/Method is defined, it has a MODULE scope, and is visible within any routine defined within that same source file, it will maintain its life for the duration of the application lifetime.

PRIVATE: Visible within the routine which declared it, and all routines called by that routine.

PUBLIC: Visible by all routines in the same application.

LOCAL and STATIC are resolved at compile time, and thus are much faster than PRIVATE and PUBLIC variables which are dynamic entities accessed by means of a runtime Symbol table. For this same reason, LOCAL and STATIC variables are not exposed to the Macro compiler, and any macro code which attempts to reference them will generate a runtime error.

Due to the dynamic nature of PRIVATE and PUBLIC variables, they can be created and destroyed at runtime, can be accessed and modified by means of runtime macros, and can be accessed and modified by Codeblocks created on the fly.

Control structures

The basic control structures include all of the standard dBase, and Clipper control structures as well as additional ones inspired by the C or Java programming languages:

Loops

The … is a sequence of one of more Harbour statements, and square bracketes [] denote optional syntax.

The HB_EnumIndex() may be optionally used to retrieve the current iteration index (1 based).

The LOOP statement restarts the current iteration of the enclosing loop structure, and if the enclosing loop is a FOR or FOR EACH loop, it increases the iterator, moving to the next iteration of the loop.

The NEXT statement closes the control structure and moves to the next iteration of loop structure.

In the FOR statement, the assignment expression is evaluated prior to the first loop iteration. The TO expression is evaluated and compared against the value of the control variable, prior to each iteration, and the loop is terminated if it evaluates to a numeric value greater than the numeric value of the control variable. The optional STEP expression is evaluated after each iteration, prior to deciding whether to perform the next iteration.

In FOR EACH, the Var variable will have the value (scalar, or complex) of the respective element in the collection value. The collection expression, may be an Array (of any type or combinations of types), an Hash Table, or an Object type.

IF statements

IF CondExp...
[ELSEIF] CondExp...
[ELSE]
...
END[IF]

… represents 0 or more statement(s). The condition expression(s) has to evaluate to a LOGICAL value.

SWITCH statements

Harbour supports a SWITCH construct inspired by the C implementation of switch().

The LiteralExp must be a compiled time resolvable numeric expression, and may involve operators, as long as such operators involve compile time static value.

The EXIT optional statement is the equivalent of the C statement break, and if present, execution of the SWITCH structure will end when the EXIT statement is reached, otherwise it will continue with the first statement below the next CASE statement (fall through).

BEGIN SEQUENCE statements

The BEGIN SEQUENCE structure allows for a well behaved abortion of any sequence, even when crossing nested procedures/functions. This means that a called procedure/function, may issue a BREAK statement, or a Break() expression, to force unfolding of any nested procedure/functions, all the way back to the first outer BEGIN SEQUENCE structure, either after its respective END statement, or a RECOVER clause if present. The Break statement may optionally pass any type of expression, which may be accepted by the RECOVER statement to allow further recovery handing.

Additionally the Harbour Error Object supports canDefault, canRetry and canSubstitute properties, which allows error handlers to perform some preparations, and then request a Retry Operation, a Resume, or return a Value to replace the expression triggering the error condition.

Alternatively TRY [CATCH] [FINALLY] statements are available on xHB library working like the SEQUENCE construct.

Procedures/Functions in Harbour can be specified with the keywordsPROCEDURE, or FUNCTION. Naming rules are same as those for Variables (up to 63 characters non case sensitive). Both Procedures and Functions may be qualified by the scope qualifier STATIC to restrict their usage to the scope of the module where defined.

The INIT or EXIT optional qualifiers, will flag the procedure to be automatically invoked just before calling the application startup procedure, or just after quitting the application, respectively. Parameters passed to a procedure/function appear in the subroutine as local variables, and may accept any type, including references.

Changes to argument variables are not reflected in respective variables passed by the calling procedure/function/method unless explicitly passed BY REFERENCE using the@ prefix.

PROCEDURE have no return value, and if used in an Expression context will produce a NIL value.

FUNCTION may return any type by means of the RETURN statement, anywhere in the body of its definition.

Development

Today Habour development is leading by Viktor Szakáts with huge collaborations and leading many components of core and contribs by Przemysław Czerpak. HBIDE and some components, specially HBQt, are developed by Pritpal Bedi. Others members send minor changes to the Sourceforge SVN repository. As 2010 Harbour development is keeping vibrant activity

Popularity

Although there is no way to measure popularity of Harbour or xBase, the TIOBE Programming Community Index As of June 2006 ranked Microsoft Visual FoxPro, a high profile dialect of xBase, on 12th position on programming languages popularity ranking. FoxPro/xBase ranked on 25th position As of August 2010. As of September 2010, the Clipper Usenetnewsgroupscomp.lang.clipper is still active. As of August 2010 Harbour figured on 16th position on weekly downloads in compiler category and 132th position on global rank.

xHarbour comparison

xHarbour is a fork of the earlier Harbour project. xHarbour takes a more aggressive approach to implementing new features in the language, while Harbour is more conservative in its approach, aiming first of all for an exact replication of Clipper behaviour and then implementing new features and extensions as a secondary consideration. It should also be noted that Harbour is supported on a wide variety of operating systems while xHarbour only really supports MS Windows and Linux 32-bit.

The Harbour developers have attempted to document all hidden behaviour in the Clipper language and test Harbour-compiled code alongside the same code compiled with Clipper to maintain compatibility.

The Harbour developers explicitly reject extensions to the language where those extensions would break Clipper compatibility. These rejections were soften recently since the new Harbour architecture allow extensions out of the core compiler.

A detailed comparison between extensions implemented in Harbour and xHarbour can be found in SVN repository of the project on SourceForge.

As of 2009–2010, Harbour has seen a huge increase in its adoption while xHarbour decline as can be seen on his mailing list